Actuators are mechanisms that move an arm or other member indirectly, rather than by hand. They find a wide variety of uses in connection with a wide variety of machines. Often, actuators must accurately position an arm or other mechanism at any requested location within the range of the actuator. One example of this requirement is in connection with an automotive heating, cooling, and ventilation system which positions a ventilation door or baffle so that hot and cold air flows are mixed together in such proportions that a desired temperature of ventilation air results.
In these applications, an actuator positioning apparatus typically obtains signals, data, or other information describing an arm's current position. The positioning apparatus may then utilize this information as an error signal that drives a feedback loop. Accordingly, an actuator is positioned by driving the arm until the error signal suggests that a desired position has been reached. However, the generation of accurate information to describe an arm's current position has posed numerous problems in prior art actuator positioning devices.
Specifically, one conventional technique uses a stationary balancing resistor having a movable wiper attached to an actuator arm. As the arm rotates or otherwise moves, the wiper moves over the resistor. Thus, a resistance changes with the position of the actuator arm, and an electrical signal with characteristics proportional to the arm's position is easily generated. This position determining technique has an advantage in that it is relatively inexpensive to implement. Unfortunately, the accuracy of this electrical signal is typically poor, and consequently actuator arm positioning accuracy is typically poor. The poor representation of actual position by the electrical signal is due, at least in part, to correspondingly poor tolerances which result from mechanically positioning an actuator arm, wiper, and resistor relative to one another.
In addition to poor accuracy, the balancing resistor technique suffers from poor reliability. Wear and the ravages of the environment, such as dust, temperature cycling, temperature extremes, moisture, and the like, further degrade the performance of this position determining technique. Thus, the poor accuracy demonstrated by this technique may be expected to become even worse over time.
Other conventional position determining techniques which improve on the accuracy and reliability of the balancing resistor technique are known. For example, various sophisticated optical and magnetic signaling and sensing devices may be coupled to an actuator arm or to a motor which drives an actuator arm to provide positioning information. In addition, more sophisticated motors, such as stepper motors, may be employed with and without feedback to achieve improved position accuracy or reliability. However, these more sophisticated techniques often achieve only marginal accuracy or reliability improvements while substantially increasing costs.